1. Field of the Disclosure
The present disclosure relates to a scanning image display device which scans laser light emitted from a light source section which includes a semiconductor laser and projects the scanned laser light on a projection target.
2. Description of the Related Art
A scanning image display device which includes a light source section, a scanning section and a projecting section has been proposed. The light source section emits laser light of intensity corresponding to an image signal. The scanning section two-dimensionally scans the laser light emitted from the light source section. The projecting section projects laser light scanned by the scanning section on a projection target and displays an image. Examples of the scanning image display device include a retinal scanning image display device of which projection target is a retina of a viewer's eye and an optical scanning display device of which projection target is a screen.
In such a scanning image display device, a semiconductor laser (i.e., a laser diode) may be used as a light source which constitutes the light source section. The semiconductor laser emits substantially no light until a value of supplied current reaches a specific threshold current value. Thus, a bias current is supplied to the light source in order to let the light source be in a standby state from the viewpoint of improving response of the light source section. That is, in a system in which an image is displayed with the laser light emitted from the light source section, a driving current corresponding to the image signal is superimposed on the bias current and is supplied to the light source.
The characteristics of the above-described semiconductor laser described above, i.e., the light source (i.e., a relationship between the current of the light source and the light output) change with, for example, heat generated during emission of the laser light, changes in the ambient temperature, and age deterioration. Then, an Automatic Power Control (APC) in which the optical output is controlled automatically is proposed as a technique to keep the optical output of the light source section constant. With the APC, for example, a bias current supplied to the light source is controlled such that the optical output of the light source becomes a predetermined value in accordance with the optical output of the light source detected by, for example, a photo diode.